1. Field of the Invention
The present invention relates to an image formation apparatus which forms an image.
2. Related Background Art
Conventionally, as an image formation apparatus which forms a color image by superimposing plural-color plane images, it has been known the color image formation apparatus such as an LBP (laser beam printer) in which a laser beam is reflected on a rotation polyhedron (i.e., polygon mirror), a photosensitive body is scanned by using the reflected laser beam to form a latent image for each line on the photosensitive body, the plane image of each color is formed by using each of M (magenta), C (cyan), Y (yellow) and BK (black) development agents on the basis of the formed latent image, and then the formed plane images are superimposed on a recording paper (or sheet) fixed on a transfer drum to form the color image.
Further, it has been also known the image formation apparatus in which the each-color plane image formed on the photosensitive body is one superimposed on an intermediate transfer body, and then the superimposed color images on the intermediate transfer body are collectively transferred onto the recording paper.
Furthermore, it has been also known the image formation apparatus in which the each-color plane image is superimposed on the photosensitive body, and then the superimposed color images are collectively transferred onto the recording paper.
In such a conventional color image formation technique, in order to prevent deterioration in color image quality due to aberration or displacement of a superimposing position of the each-color image, it gradually increases in necessity for controlling the positions when the each-color image is superimposed.
For example, U.S. Pat. No. 5,264,870 provides the structure in which the photosensitive body and a polygon motor are driven in response to the same reference signal, and the number of planes of the polygon mirror is set to be the same as the number of FG (frequency generator) pulses for each rotation of the polygon mirror. According to such the structure, even if the photosensitive body rotates many times, the scanning line by the laser beam can always trace the same position or line, whereby the plural-color plane images can be superimposed without any aberration or displacement.
However, in a case where the color image is formed by superimposing the plural-color plane images on an image support body such as the photosensitive body, the intermediate transfer body, the recording paper on the transfer body or the like, it is not yet established a method for superimposing such the plural-color plane images without any aberration or displacement.
Conventionally, in a case where movement of the image support body and rotation of a polygon scanner are respectively controlled on the basis of the reference signals which are output respectively from different oscillators, it is necessary a complicated control system for performing control such that a phase of an ITOP (image top) signal representing an image writing start position and a phase of a BD (beam detect) signal representing a rotation timing of the polygon scanner are coincided with each other, i.e., a phase in a main-scanning direction (rotation direction of polygon scanner) and a phase in a sub-scanning direction (movement direction of image support body) are coincided with each other. Therefore, it has been occurred a problem that a cost in manufacturing the apparatus raises.
Further, in a case where the polygon mirror rotates at high speed in a high-speed printer or the like, numbers of FG pulses are generated. In such the case, if the number of planes of the polygon mirror is set to be the same as the number of FG pulses such as in U.S. Pat. No. 5,264,870, a frequency of the FG pulse becomes high. Therefore, it is occurred a problem that it is difficult to control the rotation of the polygon mirror on the basis of the FG pulse.